The field of the invention is gas stream treatment.
Various industrial processes produce pollutant vapors and gases. These vapors and gases should be treated to avoid release of pollutants into the atmosphere. Other industries produce gases which include end product, e.g., paint. Recapture of end product increases the yield of a manufacturing process. Various processes have been developed to treat gas streams to serve these and other applications. Each seeks to remove gases from a gas stream.
Adsorption is a particularly useful technique. Adsorption removes a wide range of gas stream components. Adsorption process includes an adsorption step and a desorption step. During the adsorption step, the gas stream is brought into contact with sorbent in the form of granular activated carbon or zeolites. Gases adhere to the sorbent carbon or zeolite surfaces due to molecular attractive forces. The adsorbed gases are recovered during the desorption step. They are typically released by lowering pressure or by raising temperature. A typical method to raise temperature is by injection of steam. In the conventional processes, recovered adsorbed gases are often burned or converted to liquid through a refrigeration unit downstream of the desorption flow from an adsorption/desorption unit.
Other recovery techniques include material substitution, thermal oxidation, membrane filtering, absorption, and condensation. The adsorption technique is advantageous due to its wide ranging applicability, comparatively low energy requirements, and its ability to recover gas stream components which have been recaptured. Adsorption also offers selectivity. Selectivity results if the temperature in the adsorption/desorption unit can be controlled during the adsorption step because different gases will adsorb to the sorbent carbon or zeolite surfaces at different temperatures. Selectivity has become better controlled with the use of activated carbon fiber cloth (ACFC) as a sorbent, which has also yielded additional improvements to adsorption.
ACFC has twice the capacity of conventional adsorbents. It permits the rapid capture of volatile organic contaminants even when the contaminants have low concentrations in the gas stream. ACFC is ash free, which inhibits reactions with vapors such as ketone containing organic compounds and alkenes. The ACFC processes used to date follow the model of the above described adsorption/desorption processes, with ACFC felt taking the place of beds of zeolites or granular carbon.
Further improvements are realized by the invention. In the invention, an adsorption/desorption unit includes one or more elongate hollow ACFC elements. The geometric configuration of the ACFC element or elements is designed such that the elements have an electrical resistance value sufficient to permit heating of the elements by electrical current to a temperature that permits adsorption of a select gas stream constituent or constituents. The geometry also permits gas flow to penetrate the ACFC element(s). An enclosure houses the ACFC elements and is arranged to direct gas stream flow through the elements and into and out of the enclosure via gas ports. The ability to heat the elements to a desired temperature by electrical current flow allows for straightforward implementation of selective adsorption. After an adsorption step, altering the temperature of the element or elements further enables desorption.
In an embodiment of the invention, sorbate (desorbed constituent(s)) is removed from the enclosure in a gas phase. In a particularly preferred embodiment, the enclosure with the element also includes a liquid condensation outlet and desorbed constituents are directly recovered as liquid from the same unit used for adsorption and desorption. Desorbed constituents are cooled on the uninsulated chamber""s walls when gas flow rate is low during desorption and form condensate that is removed from the chamber as liquid. A system incorporating such a unit may therefore omit a separate cooling unit downstream of the adsorption/desorption unit yet still obtain liquid recovery. Local electrical heating of the ACFC during desorption removes the need to maintain a certain temperature within the desorption chamber as a whole, thus allowing condensation chilling of supersaturated desorbed constituents within the desorption chamber.
In a preferred structure, gas flow enters an ACFC element in an enclosure, and is directed through the ACFC into the enclosure volume around the ACFC. Gas flow passes through and into a second ACFC element and then exits the enclosure. The two ACFC elements are connected electrically in series. The structure is repeatable with multiple gas flows in and out of the chamber. An insulated electrical feed to the ACFC elements permits the enclosure to be metal, which is favorable for facilitating condensation.